Method of preparing laminated films while regulating moisture content

ABSTRACT

Laminated films are produced by conditioning a melt of a polyvinyl alcohol resin to a moisture content of 20 to 60 percent by weight and a temperature between 50* C. and 160* C., extruding said conditioned melt in the form of a thin film through a slit onto the surface of a roller, the roller surface temperature being adjusted to between room temperature and 100* C., passing the resulting film over at least one hot roller, and laminating said polyvinyl alcohol film with a protective film while the polyvinyl alcohol film has a moisture content of 5 to 40 percent by weight.

United States Patent Shoichl Arikawa; Kouichi Hujiwara; Kyolchlro Ikarl;Yuklo Nishlmatsu, all of Kurashlki, Japan [72] Inventors [2]] Appl. No.841,698

[22] Filed July 15, 1969 [45] Patented Dec. 7, 1971 [73] AssigneeKuraray Co., Ltd.

Kurashlki, Japan [32] Priority July 17, I968 [33] Japan [54] METHOD OFPREPARING LAMINATED FILMS WHILE REGULATING MOISTURE CONTENT 4 Claims, IDrawing Fig. '52 u.s.c| 156/244, 156/306, 156/322, l56/64 5: Int. ClB290 19/00 501 Field ol'Search 156/244,

[56] References Cited UNITED STATES PATENTS 3,470,055 9/1969 Wade156/244 Primary Examiner-Carl D. Quarforth Assistant Examiner-Stephen J.Lechert, .l r. Attorney-Stepno and Neilan hot roller, and laminatingsaid polyvinyl alcohol film with a protective film while the polyvinylalcohol film has a moisture content of 5 to 40 percent by weight.

PATENTED BEE H97! Adhesive Strength Of Laminated Film Width 0 l l l byweight) Water Content In A Polyvinyl Alcohol Film When Laminated Imvmwons SHOICHI ARIKAWA KOUICHI HUJIWARA KYOICHIRO IKARI YUKIONISHIMATSU MM 7741M ATTORNEYS METHOD OF PREPARING LAMINATED FILMS WHILEREGULATING MOISTURE CONTENT SUMMARY OF THE INVENTION The presentinvention relates to a method of preparing laminated films consisting ofa polyvinyl alcohol film (hereinafter referred to as PVA film) andaprotective film, preferably a thermoplastic film, which is firmly bondedto the PVA film.

It is a primary object of the present invention to obtain lowcost,beautiful laminated films containing the above-mentioned mentionedconstituent films, which laminated films have the desirable propertiesassociated with PVA film but do not have the disadvantages thereofparticularly with respect to water absorption. The laminated films ofthe present invention find particular utility as material for formingpackages such as bags and the like.

BACKGROUND OF THE INVENTION It is known that PVA film has excellentoxygen impermeability therelhrough and, in addition, it has verydesirable properties for use as a packaging film such as, e.g., goodtensile properties, tear strength, blowout strength, and transparencyand also the property that it may easily be formed into bags by aconventional technique using heat and/or vacuum. However, in most cases,it is difficult to use a PVA film alone as a packaging film since itabsorbs water and consequently softens when brought into contact withwater or placed in a highly humid atmosphere. In addition, theabsorption of water lowers the films printing, bagmaking and packagingproperties and increases its oxygen permeability. Therefore, when it isrequired that the packaging film have excellent oxygen impermeability,e.g., for use in packaging foodstuffs or where the package contains amoisture-containing matter or the packaged matter is subjected tosterilization by hot water, it is usual to line the PVA film with awater-resistant film on at least one surface thereof to protect the PVAfilm from water or water vapor. In these cases, although any of theknown waterproof films are useful as the protective film, it isdesirable to use .such a protective film that will give a laminated filmin combination with the PVA film which makes up for the drawbacks of thePVA film without any sacrifice of the excellent packaging properties ofthe PVA film. It is possible to obtain laminated films havingparticularly excellent quality by laminating the PVA film with aprotective film which may be a thermosetting film or a thermoplasticfilm having a low moisture permeability and good waterproofness,excellent transparency and an appropriate hardness. Examples of suitableprotective films are stretched polypropylene film, stretched polyesterfilm, polystyrene films, polyvinyl chloride, nylon and polycarbonatefilms.

PRIOR ART In prior art laminating techniques, the PVA film was laminatedwith a low moisture-permeable film such as one of those mentioned aboveby a so-called dry lamination wherein the PVA film is united with thelow moisture-permeable film by the aid of an adhesive. However, it isimpossible to attain a firm bonding between the PVA film and such lowmoisturepermeable thermoplastic film because the PVA film is formed of ahighly polar substance while the low moisture-permeable film is formedof a low polar and high crystalline substance. For this reason, theadhesives which may be conveniently used were restricted to a few kindsof adhesives and, despite their high prices, it was hard to obtain asatisfactory bonding. Also, such adhesives required use of an organicsolvent with the consequent danger of fire and, in some cases, it wasdifficult to obtain transparent laminated films due to the solubility ofthe thermoplastic film in the organic solvent.

In addition, heretofore the low moisture-permeable highlywater-resistant thermoplastic films, such as polyethylene film,polypropylene film and like quite nonpolar films, usually required anelectric discharge treatment, oxidation treatment or like surfacetreatment and, furthermore, an anchor coating for enhancement of thebonding strength. Accordingly, the procedure for the manufacture of thelaminated film was complicated and expensive. Thus, heretofore laminatedfilms of a PVA film with such a low moisture-permeable film has beenconsidered to be uneconomical for use as packaging film in spite oftheir excellent properties as a packaging film.

A so-called extrusion coating process is known in the art, e. g., thelamination of polyethylene with regenerated cellulose film in which thepolyethylene is melt extruded and simultaneously laminated with aregenerated cellulose film. However, this process has hitherto beenconsidered to be impractical for the manufacture of PVA film laminatesbecause the manufacture of PVA films is carried out by extrusion of anaqueous PVA composition and. accordingly, it is necessary to dry theextrude PVA film.

BRIEF DESCRIPTION OF THE DRAWING The drawing is a graph showing therelationship between the water content in the polyvinyl alcohol filmwhich is being laminated and the adhesive strength of the resultinglaminated film.

DETAILED DESCRIPTION OF THE INVENTION In accordance with the presentinvention it has been discovered that in the manufacture of PVA film byan extrusion process in which the PVA is extruded from a slit in theform of a film and passed through a drying step for evaporation of thecontained water, which procedure differs from the manufacture ofpolyvinyl chloride or polyethylene film by a melt extrusion process, itis possible to obtain a laminated film simultaneously with themanufacture of the PVA film by introducing a lamination step into thefilm-forming procedure and regulating the drying of the PVA film tomaintain suitable regulation of the moisture content of the PVA film.Applicants have found that very useful laminated films of the subjectconstruction may be obtained in the course of manufacturing a PVA filmby conditioning a melt of a polyvinyl alcohol resin having a moisturecontent of 20 to 60 percent by weight to a temperature of 50 to 160 C.,extruding the melt through a slit in the form ofa thin film onto aroller having a surface temperature from room temperature to C. passingthe film over one or more hot rollers so that the film achieves asemidry state having a moisture content of 5 to 40 percent by weight,and pressing the semidried PVA film against the protective film whichmay be any of the protective films referred to above. Preferably theprotective film is an untreated thermoplastic film or a thermoplasticfilm which has previously been coated with an aqueous binder, that is, a

binder containing water as a solvent or dispersion medium.

A hydrated polyvinyl alcohol resin in the form of flakes, chips orpellets, which has been adjusted to a moisture content of 20 to 60percent by weight and, optionally, to which has been added aplasticizer, a surface-active agent or other compounding ingredients, issupplied to and brought into a molten state in a screw-type meltextrusion machine. A moisture content of less than 20 percent by weightmakes it impossible to form films in the melt extrusion process, and amoisture content above 60 percent by weight makes it difficult tosmoothly extrude the melt through the lips, to stretch the extrusionbetween the lips and drying roller and to separate the drying film fromthe roller, and results in an irregularity of the resulting film andlowering of the production rate. The melt or dope is adjusted to atemperature between 50 C. and C. and extruded in the form of a thin filmthrough a slit onto the surface of a rotary drum, the surface of whichhas been smoothly plated or coated with a resin and adjusted to atemperature between normal room temperature and 100 C. A melttemperature of below 50 C. is undesirable because it causes increase inextrusion pressure, irregularity in extrusion through the lips,difficulty in stretching of the extrusion between the lips and dryingroller and results in irregularity of the surface of the resulting andlowering of efiiciency. On the other hand, a melt temperature above 160C. will cause foaming of the melt and results in irregularity of thesurface of the resulting film.

A drying roller temperature of above 100 C. is undesirable because itmakes the surface of the film irregular. The PVA film having anexcellent laminating property thus obtained is passed over a hot rolleror rollers having a surface between room temperature and 100 C. and,when it is dried to a semidry state of a moisture content of 5 to 40percent by weight, the film is laminated with the protective film toform a laminated film in which the two films are very firmly bonded toeach other.

Since, with decrease of the moisture content of the PVA film uponlamination, the stickiness of the film gradually decreases and thestructure of the films becomes tight, it is considered to be desirablefor obtaining laminated films with a high adhesion between thelaminations to laminate the PVA films with the protective film while ithas a high moisture content. However, the bonding strength between thelaminations in the resulting laminated film is not always higher whenthe PVA film is laminated while it has a high moisture content than whenlaminated after dried to a low moisture content because, if the PVA filmis laminated when it has a high moisture content, the PVA film shrinkswith elimination of the residual moisture by drying after laminationwhich causes a strain in the layer of adhesive and, thereby, the bondingstrength is lowered. From the aspect of the rate of production, thedrying speed of the resulting laminated films is lowered by thelamination with a protective film because after the lamination themoisture contained in the PVA film is allowed to vaporize from only asingle surface of the laminated film. Therefore, it is desirable tolaminate when the moisture content of the PVA film is as low aspossible. Applicants have found, as a result of investigation of therelationship between the moisture content and the bonding strength orproduction rate of the laminated films, that it is possible to obtainlaminated films exhibiting a good adhesion between the laminations byuniting under pressure a PVA film with a protective film, e.g., athermoplastic film when the PVA film has a moisture content of 5 to 40,preferably l to 35 percent by weight.

The laminated films thus obtained are in most cases, especially usefulwhere the laminated films are used for packaging hydrous matters orwhere the film is required to have a good heat-sealing property. In thelatter case, the PVA film is laminated with, e.g., a polyethylene film.The secondary protective film may be any of the known moistureimpermeable films and the lamination of the secondary protective filmmay be carried out in any of the known manners by virtue of the goodadhesion and freedom from any internal stress or strain of the laminatedfilms in accordance with the present invention.

The term protective film" herein used means any thermoplastic orthermosetting film having low moisture-permeabilities, such as ofpolyethylene, polypropylene or like polyolefin, polyester, polystyrene,nylon, polyvinyl chloride, polycarbonate or the like. In case of suchprotective films that have a relatively high polarity as indicated by ahigh solubility parameter of 14 or more, such as nylon film, there isobtained a sufiiciently firmly united laminated film without using anyadhesive providing that the moisture content of the PVA film is in therange of from to 40 percent. The adhesion between the laminations of thelaminated films in accordance with the present invention is enhanced toa great extent by subjecting the thermoplastic film to a coronadischarge treatment and, in

addition, coating with an isocyanate-based anchor coating as in the caseof the prior lamination processes. However, in accordance with thepresent invention it is possible to simplify the manufacturing processand to produce laminated films of lower price because of the concurrentpreparation of the PVA film and the laminated films.

In the practice of the method of the present invention, as the adhesivethere may be used any adhesive so long as it is prepared by using wateras a solvent because the water remaining in the adhesive layer iseliminated after lamination through the PVA film together with theresidual water in the PVA film. However, in accordance with applicantsexperiences, it is preferred for obtaining a high bonding strength touse an aqueous solution of a water-soluble high molecular weightcompound with a strong surface activity, such as partially hydrolyzedpolyvinyl esters or an aqueous emulsion of a polyvinyl acetate or vinylacetate-ethylene copolymer containing as a protective colloid apartially hydrolyzed polyvinyl ester. When using such adhesives, it ispreferred to incorporate a cross linking agent in the binder to improvethe water resistance of the adhesive.

The present invention will now be illustrated in more detail by thefollowing examples.

EXAMPLE 1 Chips of a PVA of an average saponification degree of 99.9 molwhich contains 5 percent, on the basis of PVA, of glycerol as a softenerwas melted and conditioned to a moisture content of 57 percent. The meltwas extruded at 102 C. in the form of a film through a slit under anextrusion pressure of l8 to 20 kg./cm. onto the surface of a roller thesurface of which was adjusted to a temperature of 90 C. and theresulting film was passed over hot rollers. The PVA film of a variablemoisture content thus obtained was laminated under pressure to abiaxially stretched polypropylene film to obtain a number of laminatedfilms. The polypropylene film had been previously subjected to a coronadischarge treatment and coated with N-primer (an anchor coating agentmanufactured by Dai-Nippon Ink & Chemical Industry Co., Ltd.) and thenwith 5 g./m. ofAiroo Flex 0400" (adhesive consisting of a vinylacetate-ethylene copolymer emulsion, manufactured by Air Reduction Co.,Inc. USA).

The laminated films thus obtained were subjected to friction pull testsand the results obtained were plotted in the graph as shown in theaccompanying drawing, as curve I.

In the drawing, the graph indicates the relationships between thebonding strength between the constituent films of the laminated filmsand the moisture content of the PVA film upon lamination. In the graph,the ordinate indicates the bonding strength in g./l5 mm. width and theabscissa indicates the moisture content of the PVA film on laminating inpercent by weight. The test conditions were: temperature 40 C., relativehumidity 90 percent, and pull rate 30 cm./min.

It will be seen from the figure that the bonding strength is quite lowwhen the PVA film is laminated with the biaxially stretchedpolypropylene film while its moisture content is more than 40 percent orless than 5 percent.

The roller which surface was-adjusted to a temperature of 90 C. was2,000 mm. in diameter, and each of the 15 hot rollers over which theresulting film was passed was also 2,000 mm. in diameter and whichsurfaces were adjusted to a temperature of C.

EXAMPLE 2 The procedure of example 1 was repeated twice using first apolyester film and then a polystyrene film in place of the biaxiallystretched polypropylene film. The results obtained were plotted ascurves 2 and 3 in the figure. The curves in the figure indicates thatthe bonding strength is good when the PVA film is laminated underpressure while it has a moisture content of 40 percent to 5 percent.

EXAMPLES 3 to 8 Properties of laminated films manufactured in accordancewith the present invention are shown in the following table incomparison with that of laminated films manufactured by prior artmethods (Comparative examples 1 to 3).

Example No.

Specimens Bonding Strength (g-IIS mm. width) Comparative Laminated filmsmanufactured by preparing a PVA film containing 3% of glycerol asplasticizer and laminating the PVA film at a moisture content of 35%with a biaxially stretched polypropylene film in a similar manner as inexample I.

Laminated films manufactured in the same manner as in example 3 exceptthat the polypropylene film was not subjected to corona dischargetreatment.

Laminated films manufactured in the same manner as in example 3 exceptthat the polypropylene film was not subjected to corona dischargetreatment and not coated with any anchor coating agent.

Laminated films manufactured in the same manner as in example 4 exceptthat a nylon film was used in place of the polypropylene film.

Laminated films manufactured in the same manner as in example 4 exceptthat a nylon film was used in place of the polypropylene film.

Laminated films manufactured in the same manner as in example 5 exceptthat a nylon film was used in place of the polypropylene film.

Comparative l films a polypropylene film with a PVA film ofa moisturecontent of 4% using a polyurethane based adhesive.

Comparative Example 3 Laminated films manufactured in the same manner asin comparative example 2 except that the polypropylene film was. inaddition. coated with an anchor coating. 60

The bonding strength is the friction pull strength at atmospherictemperature of 20 C. and a relative humidity of 65 percent.

What is claimed is:

l. A method of preparing a laminated film which comprises regulating themoisture content of a melt of a polyvinyl alcohol resin to a moisturecontent of 20 to 60 percent by weight, regulating the temperature of themelt to between 50 C. and C., extruding the resulting melt in the formof a thin film through a slit onto the surface of a roller, maintainingthe surface of the roller between room temperature and [00 C., passingthe resulting film over at least one hot roller and laminating saidpolyvinyl alcohol film with a protective film while the polyvinylalcohol film has a moisture content of 5 to 40 percent by weight.

2. A method according to claim 1, wherein the polyvinyl alcohol film islaminated with the protective film while it has a moisture content of IOto 35 percent by weight.

3. A method according to claim 1, wherein the polyvinyl alcohol film islaminated with the protective film coated with an adhesive containingwater.

4. A method according to claim 1, wherein said protective film is athermoplastic film.

2. A method according to claim 1, wherein the polyvinyl alcohol film islaminated with the protective film while it has a moisture content of 10to 35 percent by weight.
 3. A method according to claim 1, wherein thepolyvinyl alcohol film is laminated with the protective film coated withan adhesive containing water.
 4. A method according to claim 1, whereinsaid protective film is a thermoplastic film.